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“Module 2”
8051 Microcontroller Architecture

Module 2 :
8051 microcontroller architecture:
Microcontrollers and embedded systems,
8051 microcontroller family, 8051
architecture, Pin diagram of 8051, memory
organization of 8051

DIFFERENCE BETWEEN THE PHYSICAL STRUCTURE OF
MICROPROCESSOR AND MICROCONTROLLER

• Brief History of 8051
1. The first microprocessor 4004 was invented by Intel
Corporation.
2. 8085 and 8086 microprocessors were also invented by Intel.
3. In 1981, Intel introduced an 8-bit microcontroller called
the 8051.
4. It was referred as system on a chip because it had 128 bytes
of RAM, 4K byte of on-chip ROM, two timers, one serial
port, and 4 ports (8-bit wide), all on a single chip.
5. When it became widely popular, Intel allowed other
manufacturers to make and market different flavors of 8051
with its code compatible with 8051.
6. It means that if you write your program for one flavor of
8051, it will run on other flavors too, regardless of the
manufacturer.
7. This has led to several versions with different speeds and
amounts of on-chip RAM.

Comparison of 8051 MC and its family

8051 Microcontroller Architecture

 Features of 8051 Microcontroller
1. 4KB bytes on-chip program memory (ROM)
2. 128 bytes on-chip data memory (RAM)
3. Four register banks
4. 128 user defined software flags
5. 8-bit bidirectional data bus
6. 16-bit unidirectional address bus
7. 32 general purpose registers each of 8-bit
8. 16 bit Timers (usually 2, but may have more or less)
9. Three internal and two external Interrupts
10. Four 8-bit ports,(short model have two 8-bit ports)
11. 16-bit program counter and data pointer
12. 8051 may also have a number of special features such as
UARTs, ADC, Op-amp, etc.

What are buffer and latches in microprocessor?
• A buffer allows a signal to drive more inputs than it
would by itself, or provides input protection /
amplification. For the 8086, it's used in the output
sense, allowing internal signals to be made robust to
drive external devices.
• A latch is a circuit to accept and store one or more
bits, with a 1-to-1 input / output ratio. That is, it's
not RAM. It differs from a register in that the storage
takes place while a control input is at a particular
level (0 or 1), while a register stores the input data
upon receipt of an edge (rising or falling).

• Latches are used with 8086s to store
addresses and data, and are used instead of
registers because they maximize setup times.
That is, if data or addresses change internally
while the latch enable is active, the data
passes through immediately, while with a
register it would not be available until after
the appropriate clock transition had occurred.
The early microprocessors used every trick
they could to increase their usable speed, and
this is one of them.

• Registers are used in the CPU to store information on
temporary basis which could be data to be processed, or an
address pointing to the data which is to be fetched.
• The most widely used registers of the 8051 are A
(accumulator),
• B,
• R0-R7,
• DPTR (data pointer), and
• PC (program counter).
• All these registers are of 8-bits, except DPTR and PC.
• Also the stack pointer

• Storage Registers in 8051—
1. A register (Accumulator)
2. B register
3. Register bank
4. Data Pointer (DPTR)
5. Program Counter (PC)
6. Stack Pointer (SP)

Accumulator
• The accumulator, register A, is used for all arithmetic
and logic operations.
• If the accumulator is not present, then every result
of each calculation (addition, multiplication, shift,
etc.) is to be stored into the main memory.
• Access to main memory is slower than access to a
register like the accumulator because the technology
used for the large main memory is slower (but
cheaper) than that used for a register.

2. The "B" Register
• The "B" register is very similar to the Accumulator in
the sense that it may hold an 8-bit (1-byte) value.
• The "B" register is used only by two 8051
instructions: MUL AB and DIV AB.
• To quickly and easily multiply or divide A by another
number, you may store the other number in "B" and
make use of these two instructions.
• Apart from using MUL and DIV instructions, the "B"
register is often used as yet another temporary storage
register, much like a ninth R register.

3. Register bank
• They are a set of eight registers, namely, R0, R1 to R7.
• These registers function as auxiliary or temporary storage
registers in many operations.
• Consider an example of the sum of 10 and 20.
• Store a variable 10 in an accumulator and another variable 20 in,
say, register R4.
• To process the addition operation, execute the following
command −
ADD A,R4
• After executing this instruction, the accumulator will contain the
value 30.
• Thus "R" registers of the register bank are very important
auxiliary or helper registers.
• The Accumulator alone would not be very useful if it were not
for these "R" registers.
• The "R" registers are meant for temporarily storage of values.

• Let us take another example ---
– We will add the values in R1 and R2 together and then
subtract the values of R3 and R4 from the result.
MOV A,R3 ; // Move the value of R3 into the accumulator
ADD A,R4 ; // Add the value of R4
MOV R5,A ; //Store the resulting value temporarily in R5
MOV A,R1 ; //Move the value of R1 into the accumulator
ADD A,R2 ; //Add the value of R2
SUBB A,R5 ; //Subtract the value of R5 (which now contains R3 + R4)
– As you can see, we used R5 to temporarily hold the sum of
R3 and R4.
– Of course, this is not the most efficient way to calculate (R1 +
R2) – (R3 + R4), but it does illustrate the use of the "R"
registers as a way to store values temporarily.

4. The Data Pointer
–The Data Pointer (DPTR) is the 8051’s only
user-accessible 16-bit (2-byte) register.
–DPTR is meant for pointing to data.
–Broken down into DPH (8 bit) & DPL (8 bit)
–It is used by the 8051 to access external
memory using the address indicated by
DPTR

5.The Program Counter (PC)
• The Program Counter is a 2-byte address which tells the
8051 where the next instruction to execute can be
found in the memory.
• PC starts at 0000h when the 8051 initializes and is
incremented every time after an instruction is executed.
• PC is not always incremented by 1. Some instructions
may require 2 byte(16 bit) or 3 byte (24 bit) ; in such
cases, the PC will be incremented by 2 or 3.
• Branch, jump, and interrupt operations load the
Program Counter with an address other than the next
sequential location.
• Activating a power-on reset will cause all values in the
register to be lost.

-- What is stack?
 A stack is an array-like structure in
the memory in which data can be
stored and removed from a location
called the 'top' of the stack.
 The data that needs to be stored is
'pushed' into the stack and data to be
retrieved is 'popped' out from the
stack.

6. The Stack Pointer (SP)
– The Stack Pointer, like all registers hold an 8-bit value.
– The Stack Pointer tells the location from where the next value is to be removed
from the stack.
– When a value is pushed onto the stack, the value of SP is incremented and then
the value is stored at the resulting memory location.
– When a value is popped off the stack, the value is returned from the memory
location indicated by SP, and then the value of SP is decremented.
– This order of operation is important. First PUSH & then POP
– SP will be initialized to 07h when the 8051 is initialized (on start up).
If a value is pushed onto the stack at the same time, the value will be stored in
the internal RAM address 08h because the 8051 will first increment the value of
SP (from 07h to 08h) and then will store the pushed value at that memory
address (08h).
– SP is modified directly by the 8051 by six instructions: PUSH, POP, ACALL, LCALL,
RET, and RETI.

8051 Flag Bits and PSW Register
• The program status word (PSW) register is an 8-bit register, also
known as flag register. It contains that status bits that reflect the
current status of the CPU.
• It is of 8-bit wide but only 6-bit of it is used.
• The two unused bits are user-defined flags.
• The rest Four flags are called conditional flags, which means that
they indicate a condition which results after an instruction is
executed.
• CY (Carry),
• AC (auxiliary carry),
• P (parity),
• OV (overflow).
• The bits RS0 and RS1 are used to change the bank registers

• CY, the carry flag −
• This carry flag is set (1) whenever there is a carry out from
the D7 bit.
• It is affected after an 8-bit addition or subtraction operation.
It can also be reset to 1 or 0 directly by an instruction such as
"SETB C" and "CLR C" where "SETB" stands for set bit carry
and "CLR" stands for clear carry.
• AC, auxiliary carry flag −
• If there is a carry from D3 and D4 during an ADD or
SUB operation, the AC bit is set; otherwise, it is
cleared.
• It is used for the instruction to perform binary coded
decimal arithmetic.

• P, the parity flag −
• The parity flag represents the number of 1's in
the accumulator register only.
• If the A register contains odd number of 1's,
then P = 1; and for even number of 1's, P = 0.
• OV, the overflow flag −
• This flag is set whenever the result of a
signed number operation is too large
causing the high-order bit to overflow into
the sign bit.
• It is used only to detect errors in signed
arithmetic operations.

Example
• Show the status of CY, AC, and P flags after the
addition of 9CH and 64H in the following instruction.
MOV A, #9CH
ADD A, # 64H

Memory organization of 8051 MC

Single bit instructions
INSTRUCTION FUNCTION
SETB BIT SET THE BIT (BIT=1)
CLR BIT CLEAR THE BIT (BIT =0)
CPL BIT COMPLIMENT THE BIT (B = not BIT)
JB BIT, TARGET JUMP TO TARGET IF BIT=1
JNB BIT, TARGET JUMP TO TARGET IF BIT =0
JBC BIT, TARGET JUMP TO TARGET IF BIT =1 , THEN CLEAR
BIT

• 8051 data type
The 8051 microcontroller has only one data
type.
It is 8 bits, and the size of each register is also 8
bits.
It is the job of the programmer to break down
data larger than 8 bits (00 to FFH, or 0 to 255
in decimal) to be processed by the CPU.
The data type used by the 8051 can be positive
or negative.

8051 assembler DIRECTIVES (ALSO CALLED
PSEUDO CODE)
1. DB (define byte)
2. ORG (origin)
3. EQU (Equate)
4. END

1. The DB directive is the most widely used data directive in
the assembler.
• It is used to define the 8-bit data.
• When DB is used to define data, the numbers can be in
decimal, binary, hex, or ASCII formats.
• For decimal, the “D” after the decimal number is optional,
but using “B” (binary) and “H” (hexadecimal) for the others
is required.
• Regardless of which is used, the assembler will convert the
numbers into hex.
• To indicate ASCII, simply place the characters in quotation
marks (‘like this’). The assembler will assign the ASCII code
for the numbers or characters automatically.
• The DB directive is the only directive that can be used to
define ASCII strings larger than two characters; therefore, it
should be used for all ASCII data definitions.

2. ORG (origin)
• The ORG directive is used to indicate the
beginning of the address. The number that
comes after ORG can be either in hex or in
decimal. If the number is not followed by H, it
is decimal and the assembler will convert it to
hex

3. EQU (equate)
This is used to define a constant without occupying
a memory location.
The EQU directive does not set aside storage for a
data item but associates a constant value with a
data label so that when the label appears in the
program, its constant value will be substituted
for the label.
The following uses EQU for the counter constant
and then the constant is used to load the R3
register.

When executing the instruction “MOV R3, #COUNT”, the
register R3 will be loaded with the value 25 (notice the
# sign).
What is the advantage of using EQU?
Assume that there is a constant (a fixed value)
used in many different places in the program,
and the programmer wants to change its value
throughout.
By the use of EQU, the programmer can change it
once and the assembler will change all of its
occurrences, rather than search the entire
program trying to find every occurrence.

4.END directive
• This indicates to the assembler the end of the
source (asm) file.
• The END directive is the last line of an 8051
program, meaning that in the source code
anything after the END directive is ignored by
the assembler.

• What are label in ALP?
Labels are used to name the entry point of a
subroutine, to name the beginning of some
data, to name points in the code for
construction of control flow constructs

Rules for labels in Assembly language
1. First, each label name must be unique.
2. The names used for labels in Assembly language
programming consist of alphabetic letters in both
uppercase and lowercase, the digits 0 through 9, and the
special characters question mark (?), period (.), at (@),
underline (_), and dollar sign ($).
3. The first character of the label must be an alphabetic
character. In other words it cannot be a number.
4. Every assembler has some reserved words that must not
be used as labels in the program.
5. For example, “MOV” and “ADD” are reserved since they
are instruction mnemonics. And cannot be names of
register or instruction used in 8051 programming

NOTES: ---
1 Assembler
An assembler translates assembly language programs into
machine code. The output of an assembler is called an
object file, which contains a combination of machine
instructions as well as the data required to place these
instructions in memory.
2 Linker
Linker is a computer program that links and merges various
object files together in order to make an executable file. All
these files might have been compiled by separate
assemblers. The major task of a linker is to search and locate
referenced module/routines in a program and to determine
the memory location where these codes will be loaded,
making the program instruction to have absolute
references.

3 Loader
Loader is a part of operating system and is
responsible for loading executable files into
memory and execute them. It calculates the
size of a program (instructions and data) and
creates memory space for it. It initializes
various registers to initiate execution.
4 Cross-compiler
A compiler that runs on platform (A) and is
capable of generating executable code for
platform (B) is called a cross-compiler.

Basic instructions :
1. MOV A, #55H // LOAD VALUE 55H INTO REGISTER A
2. MOV RO, A // COPY CONTENTS OF A INTO RO
3. MOVE R3,#56H // LOAD VALUE 55H INTO REGISTER R3
4. ADD A,R2 // ADD R2 TO ACCUMALATOR (A=A+R2)
5. MOV A, #7F2H // GIVES ERROR, SOURCE & DESTINATION ARE
NOT OF SAME LENGTH
6. MOV A,#41H // COPY THE NUMBER (IMMEDIATE DATA) 41H
7. MOV A, 41 H // COPY THE DATA AT 41H RAM LOCATION

1. 8051 Microcontroller is available in a variety of
IC Packaging Types.
2. The most popular and commonly used 8051
Microcontroller Packaging is Dual in-line or DIP.
3. It is often available as a 40 – pin PDIP or Plastic
DIP IC.

8051 Microcontroller Pin Description
• Pins 1 – 8 (PORT 1):
– consists of 8 – bit bidirectional Input / Output Port.
• Pin 9 (RST):
– Pin 9 is the Reset Input Pin.
– It is an active HIGH Pin i.e. if the RST Pin is HIGH for a
minimum of two machine cycles, the microcontroller will
be reset.
– During this time, the oscillator must be running.

• Pins 10 – 17 (PORT 3):
 Pins 10 to 17 form the PORT 3 pins of the 8051 Microcontroller.
 PORT 3 also acts as a bidirectional Input / Output PORT.
 Additionally, all the PORT 3 Pins have special functions. The following table
gives the details of the additional functions of PORT 3 Pins.

• Pins 18 & 19:
– Pins 18 and 19 i.e. XTAL 2 and XTAL 1 are the pins for
connecting external oscillator.
– Generally, a Quartz Crystal Oscillator is connected
here.
• Pin 20 (GND):
– Pin 20 is the Ground Pin of the 8051 Microcontroller.
– It represents 0V and is connected to the negative
terminal (0V) of the Power Supply.

• Pins 21 – 28 (PORT 2):
– These are the PORT 2 Pins of the 8051
Microcontroller.
– PORT 2 is also a Bidirectional Port i.e. all the PORT
2 pins act as Input or Output.
– Additionally, when external memory is interfaced,
PORT 2 pins act as the higher order address byte.

• Pin 29 (PSEN):
– Pin 29 is the Program Store Enable Pin (PSEN).
– Using this pins, external Program Memory can be
read.
• Pin 30 (ALE/PROG):
– Pin 30 is the Address Latch Enable Pin.
– Using this Pins, external address can be separated
from data (as they are multiplexed by 8051).
– During Flash Programming, this pin acts as program
pulse input (PROG).

• Pin 31 (EA/VPP):
• Pin 31 is the External Access Enable Pin i.e.
allows external Program Memory.
• Code from external program memory can be
fetched only if this pin is LOW.
• For normal operations, this pins is pulled HIGH.
• During Flash Programming, this Pin receives 12V
Programming Enable Voltage (VPP).

• Pins 32 – 39 (PORT 0):
– Pins 32 to 39 are PORT 0 Pins.
– They are also bidirectional Input / Output Pins
– In addition to acting as I/O PORT, PORT 0 also acts
as lower order address/data bus when external
memory is accessed.
• Pin 40 (VCC):
– Pin 40 is the power supply pin to which the supply
voltage is given (+5V).

 Some of the applications of 8051 Microcontroller are mentioned
below:
1. Consumer Appliances (TV Tuners, Remote controls, Computers,
Sewing Machines, etc.)
2. Home Applications (TVs, VCR, Video Games, Camcorder, Music
Instruments, Home Security Systems, Garage Door Openers,
etc.)
3. Communication Systems (Mobile Phones, Intercoms, Answering
Machines, Paging Devices, etc.)
4. Office (Fax Machines, Printers, Copiers, Laser Printers, etc.)
5. Automobiles (Air Bags, ABS, Engine Control, Transmission
Control, Temperature Control, Keyless Entry, etc)
6. Aeronautical and Space
7. Medical Equipment
8. Defense Systems
9. Robotics
10. Industrial Process and Flow Control
11. Radio and Networking Equipment
12. Remote Sensing

Oscillator Circuits
1. The 8051 requires a crystal oscillator circuit.
2. The oscillator circuit usually runs around 12 MHz(
11.0592Mhz specifically)
3. Each machine cycle in the 8051 is 12 clock cycles,
giving an effective cycle rate at 1 MHz (for a 12 MHz
clock) to 3.33 MHz (for the maximum 40 MHz clock).
4. The oscillator circuit generates the clock pulses so
that all internal operations are synchronized.
5. One machine cycle has 6 states. One state is 2 T-
states. Therefore one machine cycle is 12 T-states.
6. Time to execute an instruction is found by
multiplying C by 12 and dividing product by Crystal
frequency.
7. T=(C*12d)/crystal frequency

MACHINE CYCLE CALCULATION
FORMULA :---
MACHINE CYCLE FREQUENCY = 1/12 XTAL
1.FIND THE MACHINE CYCLE FOR—
a) XTAL = 11.0592 MHz = 1.085 microsec
b) XTAL = 16 MHz = 0.75 microsec

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